The present invention relates to an economical surge absorber which has a prolonged life and a good annealing characteristic as well as a very small discharge lag and a small electrostatic capacity and is excellent in cutting off a follow current.
Conventional surge absorbers comprise a gap type lightning arrester, ZnO varistor and the like. The gap type lightning arrester in which the electrodes are opposed to each other through the gap has a large discharge lag and is lacking in cutting off a follow current and further has a sparkover voltage different in the light and the dark. ZnO varistor is composed of ceramics consisting of ZnO and a small quantity of impurity such as Bi.sub.2 O.sub.3, in which nonconductive Bi.sub.2 O.sub.3 concentrates in the boundary of crystal grains of conductive ZnO, thereby generating varistor characteristics. However, ZnO varistor has a large electrostatic capacity and requires a fuse for safety, because it becomes short-circuited when electrically destroyed and is limited in use by its high residual voltage.
Accordingly, the applicant has proposed a two stage discharge type surge absorber, as an improved one of these conventional surge absorbers above mentioned, in Japanese Patent Applications No. 22185/1975 and No. 82598/1975. This two stage discharge type surge absorber comprises a plurality of areas of conductive thin film composed of carbon thin film, conductive paint thin film or metal thin film formed on the surface of a molded insulating body composed of mullite porcelain and the like having a small dielectric constant separated from each other by an air gap formed by marking off the conductive thin film with a streak having a very narrow width and making the surface of the molded insulating body exposed, electrodes fixed to the thus separated conductive thin film and, if required, a gas sealed between the electrodes.
In the above mentioned construction, when a surge voltage is applied to the electrodes, at first the electric field is concentrated to the air gap between the separated conductive thin films, as the separated conductive thin film is opposed to the adjacent one through the air gap respectively. The thickness of the conductive thin film is usually 0.1-100 .mu.m, so the concentration grade of the electric field is large, and as the air gap in this case is supported by the surface of the molded insulating body having a relative inductive capacity of 6-10, the inductive capacity of the air gap is 6-10 times larger than that of a penetrating air gap, so the electric field is more and more concentrated to the air gap to emit easily electrons due to the electric field through the air gap, thus there is generated a first stage discharge.
Next, the electrons emitted by the first stage discharge collide with neighboring gas molecules and ionize them. New electrons flying out from the gas molecules in their ionization ionize further gas molecules. This phenomenon is repeated to accelerate the ionization of gas molecules, finally the insulation of gas is destroyed, thereby generating a gas discharge between the electrodes as a second stage discharge. Therefore, this second state discharge is extremely rapid, and is small in discharge lag. This gas discharge transfers from a glow discharge to an arc discharge as the surge current increases, but the main constituent of this gas discharge is a creeping discharge which is generated along the surface of the conductive thin film. As this creeping discharge is remarkably generated especially at an initial stage of the second stage gas discharge, it is effective in accelerating the second stage gas discharge and is very small in discharge lag. The relation between the first stage discharge and the second stage discharge can be understood by equivalent circuits shown in FIG. 9(a) and (b). Namely, the first stage discharge is only an electron emission through the streak 3 and in the second stage discharge, the main constituent of the discharge is a gas discharge which is mainly composed of a creeping discharge generated between the electrodes 4, 4 parallel with the streak 3.
Thus, the surge absorber based on a two stage discharge mechanism, that is, the two stage discharge type surge absorber has the following features, as compared with the conventional gap type lightning arrester and ZnO varistor:
(1) discharge lag is small.
(2) cutting off of a follow current is good.
(3) sparkover voltage is stable.
(4) electrostatic capacity is small.
(5) short-circuit is not generated when electrically destroyed.
(6) residual voltage is small.
However, the two stage discharge type surge absorber above-mentioned was still insufficient in surge absorbing characteristics, especially unsatisfactory in life- and annealing characteristics.